Membrane: Structure And Function

Membrane: Structure And Function Chapter title: Membrane Structure and Function. The ability of the cell to discriminate in its chemical exchanges with the environment is fundamental to life, and it is the plasma membrane that makes this selectivity possible. Membrane The membranes that are found within cells (plus the plasma membrane surrounding cells) consist of phospholipids (and other lipids plus membrane proteins) arrayed by hydrophobic exclusion into two-dimensional fluids known as known as lipid bilayers Phospholipids Phospholipids are amphipathic molecules meaning that they have both a hydrophobic and a hydrophilic end Lipid bilayer Phospholipids can exist as bilayers in aqueous solutions The hydrophobic portion of the phospholipid is shielded in middle of these bilayers The hydrophilic portion is exposed on both sides to water Lipid bilayers are held together mainly by hydrophobic interactions (including hydrophobic exclusion) Fluid mosaic model The plasma membrane contains proteins, sugars, and other lipids in addition to the phospholipids The model that describes the arrangement of these substances in and about lipid bilayers is called the fluid mosaic model Basically, membrane proteins are suspended within a two-dimensional fluid that in turn is made up mostly of phospholipids Cholesterol Cholesterol, a kind of steroid, is an amphipathic lipid that is found in lipid bilayers that serves as a temperature-stability buffer At higher temperatures cholesterol serves to impede phospholipid fluidity At lower temperatures cholesterol interferes with solidification of membranes (e.g., cholesterol functions similarly, in the latter case, to the effect of unsaturated fatty acids on lipid-bilayer fluidity) Cholesterol is found particularly in animal cell membranes Membrane proteins Proteins are typically associated with cell membranes Integral membrane proteins are typically hydrophobic where they interact with the hydrophobic portion of the membrane or hydrophilic where they interact with the hydrophilic portion of the membrane and overlying Functions of membrane proteins Functions of membrane proteins include: Transport of substances across membranes Enzymatic activity cell communication Cell-to-cell joining Attachment to the cytoskeleton and extracellular matrix Selective permeability Lipid bilayers display selective permeability In general, intact lipid bilayers are permeable to: Hydrophobic molecules (including many gasses) Small, not-ionized molecules   Simultaneously, lipid bilyaers are NOT permeable to: Larger, polar molecules (e.g., sugars) Ions, regardless of size Thus, lipid bilayers are selectively permeable barriers that allow the entry of small or hydrophobic molecules while blocking the entry of larger polar or even small charged substances Transport across membranes Movement across membranes is important, for instance as a means of removing wastes from a cell or bringing food into a cell Categories of substance transport across membranes include: Passive transport Facilitated diffusion Active transport (including cotransport) Endocytosis, phagocytosis, and exocytosis, also considered below, technically are not mechanisms of movement of substances across lipid bilayers (though these do represent movements of substances into and out of cells; to be movement across the euakaryotic cell membrane, a substance must actually pass through an endomembrane lipid bilayer) Note that in considering transport across membranes we will once again confront the concept of movement away from or towards equilibrium, i.e., endergonic and exergonic processes There are three basic types of movement across membranes: simple diffusion, passive transport, and active transport: Simple diffusion Simple diffusion is the movement of substances across lipid bilayers without the aid of membrane proteins This image (below) shows how substances move through membranes, regardless of net direction and concentration gradients: This image (below) shows how substances net move through membranes in the direction of their concentrations gradients (i.e., with their concentration gradients)-note that regardless of how net movement is accomplished, all simple diffusion across membranes occurs in the manner illustrated above, i.e., it is a process that is driven by the random movement of molecules: This figure (below) indicates the kinds of molecules that are capable of moving across membranes via simple diffusion: Passive transport Passive transport is the term used to describe the diffusion (as well as what is termed facilitated diffusion, below) of substances across lipid bilayers Passive transport is a consequence of movement through the lipid bilayer (whether by diffusion through the membrane or with movement across facilitated by an integral membrane protein) a concentration gradient thereby contrasting with active transport Down the concentration gradient Diffusion is a random process that tends to result in the net movement of substances from areas of high concentration to areas of low concentration This includes movement from one side of a permeable lipid bilayer to the other from the higher concentration side to the lower concentration side (i.e., passive transport) Movement from high to low concentration areas is described as going down its concentration gradient. The direction of movement of substances across lipid bilayers by passive transport is controlled by concentration gradients Osmosis Movement of water across selectively permeable membranes down the water concentration gradient is called osmosis Note that this is movement toward equilibrium (exergonic process) Tonicity (isotonic, hypertonic, hypotonic) Picture a membrane separating two solutions, one side with a higher solute concentration than the other The side with the higher solute concentration is said to be hypertonic The side with the lower solute concentration is said to be hypotonic (I keep track of the difference by recalling that a hypodermic syringe is so named because the tip of the needle is placed beneath the dermis, i.e., under the skin; a hypotonic solution has a solute concentration that is beneath, i.e., lower than that of the reference solution) If both sides have the same solute concentration, they are said to be isotonic Animal cells and tonicity Normally animal cells are bathed in an isotonic solution Placement of an animal cell in a hypertonic solution causes the cell to shrink (i.e., water is lost from the cell by osmosis) Placement of an animal cell in a hypotonic solution causes it to take on water then burst (lyse, i.e., die) (water is gained by the cell, lost from the environment bathing the cell, both by osmosis) Turgidity Normally a plant cell exists in a hypotonic environment The hypotonicity causes the plant cytoplasm to expand However the plant cell does not lyse and this is due to the presence of its cell wall This conditions is known as turgidity (i.e., the pressing of the plant plasma membrane up against its cell wall) Plant cells prefer to display turgidity Plasmolysis A plant or bacterial cell placed in a hypertonic environment will show a shrinkage of its cytoplasm This shrinkage is called plasmolysis At the very least plasmolysis will inhibit growth Often plasmolysis will lead to cell death This is the principle upon which foods are preserved in highly osmotic solutions (e.g., salt or sugar); such solutions impede most microbial growth Flaccidity Plant cells bathed in isotonic solutions will fail to display turgidity Instead they display flaccidity At a whole-organismal level, flaccidity is otherwise known as wilting Transport proteins Substances (e.g., sugars) that are not permeable through lipid bilayers may still cross via membrane-spanning transport proteins Facilitated diffusion Facilitated diffusion is the movement of a substance across a membrane via the employment of a transport protein, where net movement can only occur with the concentration gradient, is called facilitated diffusion The key thing to keep in mind is that facilitated diffusion, in contrast to other mechanisms of transport-protein-mediated membrane crossing, does not require any input of energy beyond that necessary to place the protein in the membrane in the first place (i.e., facilitated diffusion is an exergonic process) Passive versus active transport Two general categories of transport across membranes exist: Those that dont require an input of energy (passive transport, simple diffusion, facilitated diffusion) Those that do require an input of energy (active transport) Passive Transport Active Transport Concentration gradient With (Down) Against (Up) Without Integral Protein Yes (Simple Diffusion) No With Integral Protein Yes   Yes Examples Small or Hydrophobic Substances, Osmosis(by simple diffusion) or Not-Small or Charged Substances (by facilitated diffusion) Cotransport, Proton Pump, Sodium-Potassium Pump Active transport Active transport is the movement of substances across membranes against their concentration gradients Moving things against their concentration gradients requires an expenditure of energy (i.e., it is an endergonic process) This energy can be in the form of ATP (e.g., sodium-potassium pump) This energy can also be in the form of electrochemical gradients (i.e., cotransport) Note that the movement of substances by active transport is in a direction that is away from equilibrium Sodium-potassium pump One means by which cells actively transport substances across membranes is via the sodium-potassium pump The sodium-potassium pump is important especially in animal cells, and is the means by which the sodium-potassium electrochemical gradient is established by these cells Proton pump The sodium-potassium pump is the means by which animal cells generate membrane potentials In bacteria, plants, and fungi, proton pumps play the same role The proton pump is simply ATP-driven active transport in which the substance pumped across the membrane is a hydrogen ion Cotransport Much of the active transport accomplished by a cell isnt directly powered by ATP Instead, much active transport is powered by membrane potentials (i.e., electrochemical gradients) Such electrochemical-gradient-driven active transport is called cotransport In cotransport, one substance, such as a sugar, is driven up its concentration gradient while a second substance, e.g., sodium ions or protons, are allowed to fall down their electrochemical gradient; the energy gained from the latter is employed to power the former (i.e., energy coupling) Endocytosis Endocytosis is a general category of mechanisms that move substances from outside of the cell to inside of the cell, but neither across a membrane (technically) nor into the cytoplasm (again, technically speaking) Instead, substances are moved from outside of the cell and into the lumens of endomembrane system members To enter the cytoplasm an endocytosed substance must still be moved across the membrane of the endomembrane system, e.g., following their digestion (typically hydrolysis) to smaller molecules Examples include: phagocytosis, pinocytosis, and receptor-mediated endocytosis Phagocytosis Phagocytosis is the engulfing of extracellular particles is achieved by wrapping pseudopodia around the particles, thus internalizing the particles into vacuoles Amoebas employ phagocytosis to eat Most protozoa obtain their food by engulfing, i.e., via some form of endocytosis The advantage of endocytosis as a mechanism of food gathering has to do with minimizing the volume within which digestive enzymes must work in order to digest food, i.e., the engulfed food particle Cells in our own bodies, called phagocytes and macrophages employ phagocytosis to engulf (and then destroy) debris floating around our bodies as well as to engulf and destroy invading bacteria Pinocytosis Pinocytosis is the engulfing of liquid surrounding a cell This is how developing ova obtain nutrients from their surrounding nurse cells (ova are very large cells so have surface-to-volume problems-pinocytosis solves the problem of nutrient acquisition by allowing nutrients to be obtained across many internal membranes rather than being limited to crossing the plasma membrane) Receptor-mediated endocytosis Receptor-mediated endocytosis involves the binding of extracellular substances to membrane-associated receptors, which in turn induces the formation of a vesicles Exocytosis Exocytosis is more or less the mechanistic opposite of endocytosis